Battery-powered devices (e.g., consumer electronic devices, electric and hybrid automobiles, etc.) are charged from a power source (e.g., AC power outlet) through a charging device. The charging device couples the battery to the power source through an adaptor. The cord extending between the power source and the battery-powered device can take up space. In situations where multiple devices require charging, each with their own charger and cord, the charging area can become cramped and inconvenient.
Approaches are being developed that use over-the-air or wireless power transmission between a transmitter and a receiver coupled to the electronic device. Wireless power transmission using inductive coils is one method considered as an un-tethered method for transferring power wirelessly through a coupled wireless power signal. In wireless power transmission, power is transferred by transmitting a wireless power signal through a transmit coil. On the receiver side, a receive coil may couple with the transmit coil through the wireless power signal, thus, receiving the transmitted power wirelessly. The distance between the transmitter coil and receive coil, at which efficient power transfer can take place, is a function of the transmitted energy, the distance, and the alignment of the power transfer coils. The coupling coefficient (k) is a function of the distance and alignment between the coils, the coil sizes, and materials. The power conversion efficiency (e.g., coupling factor, coupling quality) may be significantly improved if the coils are sized and operated at such a frequency that they are physically within the so-called “near-field zone” of each other.
Wireless power transmitters sometimes have a plurality of transmit coils so that the wireless power receiver has multiple locations (e.g., within a charging pad) to draw power from. Typically, only one of the transmit coils transmits at a given time in order to conserve power as well as reduce potential adverse effects on the wireless power receiver and/or foreign objects.
Conventional low voltage transmitters may require high current to deliver power at a suitable level (e.g., 5 W). High current results in a stronger magnetic field (B-field), which in turn can couple (i.e., parasitic coupling) into an adjacent unused coil, which may generate high voltages and current in portions of the circuits that are not being used. As a result, the efficiency of the wireless power transfer may be reduced as energy may be lost from the system through the parasitic coupling with the unused coil, and components with higher voltage ratings may be needed to prevent damage to the electronic subsystems that comprise the wireless power transfer system.